Electric power steering apparatus

Abstract

The invention provides an electric power steering apparatus, in which one of a pair of switching elements that has been PWM controlled is turned off and the other switching element is PWM controlled instead when a steering wheel is returning to its neutral position, thereby allowing the steering wheel to return smoothly and preventing a control hunting.

What is claimed is: 
     
       1. An electric power steering apparatus for assisting a steering force by driving a motor in response to torque supplied to a steering wheel that can be turned in either direction relative to a neutral position, comprising: a bridge circuit including said motor, a second and a third switching element for driving said motor in a clockwise direction, and a first and a fourth switching element for driving said motor in a counter-clockwise direction;   decision means for deciding whether said steering wheel is being returned to its neutral position; and   means for performing PWM control on said third switching element in accordance with the torque supplied to the steering wheel and turning off said second switching element when said decision means decides that the steering wheel is being returned to its neutral position as said motor is driven in the clockwise direction and for performing PWM control on said fourth switching element in accordance with the torque supplied to the steering wheel and turning off said first switching element when said decision means decides that the steering wheel is being returned to its neutral position as said motor is driven in the counter-clockwise direction.   
     
     
       2. An electric power steering apparatus according to claim 1, wherein said decision means includes: a resistance connected with said motor in series at a bridging portion in said bridge circuit;   a differential amplifier for obtaining a difference between a motor driving current detected by said resistance and a target value of the motor driving current in accordance with the torque supplied to the steering wheel;   means for deciding whether polarities of said target value of the motor driving current and a differential output of said differential amplifier are different; and   means for detecting change of the polarity of said differential output, and   said decision means decides that the steering wheel is being returned to its neutral position when said means for deciding whether the polarities are different decides that the polarities are different and said means for detecting the change of the polarity decides that the polarity is changed.